It has long been desired to find an inexpensive means of making alkyl methacrylate and alkyl acrylates, basic monomers for the formation of acrylic resins, as well as acrylic and methacrylic acids. Conventionally, methyl methacrylate is prepared by reacting acetone and hydrogen cyanide to form cyanohydrin; dehydrating the cyanohydrin in the presence of sulfuric acid to form methacrylamide sulfate; and finally reacting the sulfate with methanol and sulfuric acid to form the desired methyl methacrylate. Because of the high cost of the raw materials and the need to dispose of by-product ammonium sulfate, this process is deficient.
Other proposed processes for making methyl methacrylate and methacrylic acid involve isobutylene oxidation, ammoxidation, epoxide formation and t-butanol oxidation. These processes also suffer from high capital costs and, in some cases, raw material costs.
In the present invention it is proposed to prepare acrylic acid, methacrylic acid and corresponding esters from olefins and carbon monoxide in the presence of a catalyst complex composed of one mole of BF.sub.3 and one mole of either water or an alcohol, as the case may be. Where the alcohol is methanol and the olefin propylene, the product is methyl isobutyrate. The latter may be dehydrogenated to prepare the methyl methacrylate. In this process the end products are formed from readily available raw materials, no by-products are formed, and capital costs are economically attractive.
In the past it has been proposed that methacrylic acid (or methyl isobutyrate) be prepared from propylene by reacting propylene with carbon monoxide and water (or methanol) and a number of catalysts have been claimed to effect this reaction. For example, U.S. Pat. No. 3,579,511 describes the reaction of olefins with carbon monoxide and water in the presence of catalyst compositions essentially comprising iridium compounds and complexes together with an iodide promoter. Similarly German OLS No. 2,739,096 describes the carbonylation of propylene in the presence of palladium salts. Unfortunately, the use of noble metals as catalyst is costly and this aspect makes these approaches economically unattractive.
Carbonylation of higher olefins, i.e., isobutylene and above, with carbon monoxide and water in the presence of acid catalyst has long been known. The reaction, known as the Koch synthesis, has been commercialized by Exxon [Hydrocarbon Processing 44, 139 (1965)] and also described in French Pat. No. 1,252,675 granted to Shell International. A variety of acid catalysts are described including sulfuric and phosphoric acid and hydrated boron trifluoride. The Exxon process uses boron trifluoride dihydrate and the French patent prefers a mixture of phosphoric acid and boron trifluoride. In both instances the feed used was isobutylene and the product was primarily 2,2-dimethyl propionic acid. Only in the case of high molecular weight olefins, particularly those branched at the double bond, was it believed that one could form carboxylic acids having only one more carbon than the olefin. This was predicated on the finding that, in order to obtain high conversions, the less reactive lower olefins, i.e., ethylene and propylene, could only be carbonylated under severe reaction conditions, and that under these conditions the major reaction was the polymerization of the olefin.
Pawlenko in Chemie Ing. Techn. 40, 52 (1968) and German Pat. No. 1,226,557 amply supports the above observations. After recognizing that under the original conditions of the Koch synthesis ethylene and propylene cannot be carbonylated and that under severe conditions in the presence of boron trifluoride hydrate, ethylene forms the high molecular weight ethyl ester of alpha-methyl-alpha-ethylbutyric acid, Pawlenko proposes the use of a [H.sub.3 O][BF.sub.4 ] catalyst. With this catalyst, he reports that ethylene and propylene under comparatively mild conditions form primarily carboxylic acids having over seven carbon atoms. Similarly, using [CH.sub.3 OH][BF.sub.3 ] as the catalyst, methyl esters of C.sub.6, C.sub.8 and C.sub.12 acid were obtained.
For the above reason, much of the prior art relating to the Koch synthesis is limited to higher olefin feeds. See, for example, U.S. Pat. No. 3,349,107; Izv. Akad. Nauk., SSSR Ser. Khim. 1970 (2) 424; Ibid. 1970 (7) 1673; and Ibid. 1972 (2) 428.
In the foregoing prior art, while boron trifluorides are generally described, most of the work with the higher olefins used such complexes wherein the number of moles of water or alcohol present is two or more times the number of moles of the boron trifluoride. See, for example, the above-cited Hydrocarbon Processing article and the three USSR references. In Moller, Brennstoff-Chemie, 45, 129 (1964), it is noted that all mixtures of BF.sub.3 hydrate catalyst may be used which contain more water than BF.sub.3.H.sub.2 O up to 2 moles of water for each mole of BF.sub.3. A similar stipulation is made for the methanol complex. The catalyst BF.sub.3.CH.sub.3 OH is only said to be useful for certain cyclic olefins with the added observation that such catalyst is of little interest because it cannot be recovered in a useable form. Furthermore, this reference, too, is limited to the carbonylation of higher olefins, C.sub.5 to C.sub.12.